This proposal will test the hypothesis that the successful elimination of Brugia malayi by the murine host requires the concerted action of the adaptive and innate immune systems. We hypothesize that (a) the adaptive immune system in immunocompetent mice responds to immunogen(s) of the L3 larvae to (b) up-regulate the activity of the innate immune system, probably by the secretion of cytokines like TNF-alpha and IFN-gamma. We further hypothesize that (c) components of the immune system (such as macrophages) then secrete a variety of chemical mediators, including, but not limited to nitric oxide, that (d) inflict damage on the infectious agent. We will examine; 1. The early immune response of normal mice to challenge infection: We will examine the (a). dynamics of the immune response in draining lymph nodes and the spleen, by flow microfluorimetry, using reagents to quantitate B cells, alpha beta T lymphocytes (CD4 T and CD8 cells), gammadelta T lymphocytes, and the expression of activation markers such as CD44 and CD45Ro. (b). kinetics of response of various Vbeta subsets of T lymphocytes, both in the CD4 and CD8 subsets ina the draining lymph node and in the spleen/ (c). evolution of cytokine secretion by draining lymph node cells, both by direct ELISA assays, as well as semi-quantitative RT- PCR. 2. The mechanism of anti-filarial immunity in MHC class II and TCRalphabeta knock-out mice that are resistant to challenge infection. We will examine the (a). kinetics of worm elimination in these knock-out mice in comparison to normals. (b). the cell population dynamics in draining lymph node using cell surface markers as indicated above in Specific aim #1. (c). the evolution of cytokine secretion by draining lymph node cells, both by direct ELISA assays, as well as semi-quantitative RT-PCR. 3. The role of nitric oxide in host defense against filarial infection. We will examine (a) the growth of B. malayi in mice in which the inducible nitric oxide synthase (iNOS) gene has been inactivated by homologous recombination. (b) the influence of alleles at the Nramp and iNOS loci on murine susceptibility to B. malayi. (c). the role of cytokines controlling the induction of nitric oxide in the resistance of NOD/LtSz-scid mice to B. malayi.